1. Field of the Invention
The present invention relates to an electromagnetic brake assembly of the type in which an electromagnetic coil is energizable to bias a braking mechanism into or out of a releasable engagement with a movable surface in order to prevent relative movement between the braking mechanism and the surface, and to a power supply for energizing the electromagnet with a direct current.
2. Description of the Prior Art
Electromagnetic brake assemblies are known wherein a braking mechanism, such as a friction plate, is mounted for movement between a first position at which it is located in braking engagement with a movable surface and a second position at which it is located out of braking engagement with the movable surface, and having an electromagnet, energizable by a direct current source to bias the braking mechanism to one or other of the first and the second positions. One such brake assembly is described in U.S. Pat. No. 5,148,467 where it is described as being employed to arrest the rotational movement of an X-ray equipment holder in order to lock it at a desired arbitrary position. The disclosed electromagnetic brake assembly has an electromagnet and co-operating braking mechanism. When the electromagnet is energized to generate a magnetic force the braking mechanism is caused by that force to move to the first position where it presses against a rotatable disk of the holder. When the electromagnet is de-energized, a return spring returns the braking mechanism to the second position allowing rotation of the rotatable disk. A further magnetic brake assembly is described in U.S. Pat. No. 4,759,048 where again it is employed to arrest rotational movement of an X-ray equipment holder. Here an electromagnet is energizable to attract a braking mechanism to cause it to move to the second position in which a rotatable disk becomes unlocked. When the electromagnet is de-energized the braking mechanism is urged under the influence of a return spring to the first position where it engages the disk to lock its rotational movement. In both configurations of the electromagnetic brake assemblies described above the return spring must be made strong enough to counter the often-substantial residual magnetism that remains with the electromagnet after its de-energization. Unfortunately, this results in the need for a relatively more powerful electromagnet to generate a magnetic force sufficient to additionally counter the spring force before the braking mechanism can be caused to move.